Programmable processing devices are today used in a wide variety of applications, and typically comprise program controlled functional units integrated with program memory and/or microprogram memory on a single integrated circuit chip. It is generally required to allow verification of the program data stored on chip. However, transferring the entire memory contents to the test equipment for verification may require excessive dedicated test hardware on-chip, and requires expensive test equipment. To reduce this problem, devices having built-in self-test features have been developed in which the stored data are compressed on-chip into a so-called signature word which can be used to verify the entire memory contents with an adequate degree of certainty.
An example of a processing device with a built-in memory self-test facility, as set forth in the opening paragraph, has been described by M. M. Ligthart, P. Baltus and M. Freeman in "RPST: A ROM based Pseudo-exhaustive Self Test approach", IEEE Proc. ITC 1987, pp. 915-922. Another example is described by J. Kuban and H. Bruce in "The MC6804P2 built-in self-test", IEEE Proc. ITC 1983, pp. 295-300. In the known devices, only the generated signature word needs to be transferred off-chip, to be compared to the expected signature word. There is still a need for external knowledge of the expected signature word, and a need for dedicated data paths for communication of the signature word.